Demiral, 21, joined Sassuolo from Alanyaspor in January – initially on loan with an obligation to buy – before making 14 Serie A appearances.
The centre-back, a four-time Turkey international, has now joined the Serie A champions, signing a five-year deal.
“Juventus Football Club S.p.A. announces that the agreement with U.S. Sassuolo Calcio S.r.l. for the definitive acquisition of the registration rights of the player Merih Demiral has been finalised for consideration of €18million payable in four financial years,” a Juve statement read.
“Juventus and the player have signed a five-year contract of employment until 30 June 2024.”
Demiral also scored two Serie A goals in his 14 matches as Sassuolo finished 11th in the table.
His signing continues Juve’s strengthening, with Aaron Ramsey, Luca Pellegrini, Adrien Rabiot and Gianluigi Buffon having signed for Maurizio Sarri’s side ahead of 2019-20.